(a) Technical Field
The present invention relates to a pulse power generator. More particularly, the present invention relates to a pulse power generator which can minimize a voltage droop generated during the application of a high voltage pulse by additionally including a simple circuit configuration without a significant increase in cost and volume.
(b) Background Art
Generally, a high voltage pulse generating circuit uses apparatuses such as a plasma generator (PSII etc.) or various kinds of test equipment, which require a high voltage as a load. A typical high voltage pulse generating circuit has many limitations in terms of the lifespan of the apparatus, the variation of the pulse width, the increase of the pulse repetition rate, the control of the pulse voltage, and the necessity of the DC high voltage power source.
For example, there have been used various pulse generating circuits utilizing gas discharging switches such as a spark gap and a vacuum tube. However, these methods have limitations in that the lifespan of the apparatus is short and the pulse width is impossible to control. Also, there are limitations in that it is difficult to increase the pulse repetition rate and a DC high voltage power supply circuit is needed.
Also, since a method of using a pulse transformer is difficult to obtain a rapid rise time of a pulse due to the leakage inductance of the transformer, and a reset circuit needs to be added due to the self-saturation of the transformer, the circuit becomes complex, noise occurs and the pulse width is difficult to extend.
In the Marx pulse generator, efforts have been made to use an Insulated Gate Bipolar Transistor (hereinafter, referred to as IGBT) that is a semiconductor switch instead of a spark gap switch.
The IGBT is permanent, and can overcome limitations of a mechanical switch used in a typical Marx pulse generator because it becomes possible to control the pulse repetition rate and the pulse width. However, the IGBT also has operational limitations such as switching driving and uniform voltage distribution, causing the limitations of the product reliability.
Core technology of the pulse generator using the IGBT is to overcome the voltage and current rating of a switch. IGBT has a low voltage and current rating unlike a typical gas discharging switching.
Thus, one IGBT is not used instead of one spark gap switch, but a plurality of IGBTs are connected in series as many as required to endure the voltage rating to be turned on/off. In this case, when IGBTs turn on/off, a voltage unbalance may be easily generated due to a difference of the driving timing, and thus may immediately damage the IGBTs when overpassing the voltage rating.
Also, when IGBTs are driven in series, each switch needs an independent driving power. In this case, the insulation strength of the independent driving power has to become greater at the upper portion of a series switch configuration. Accordingly, an insulation technology of the driving power is known as one of the most difficult technologies in the high voltage driving.
In a related art regarding a technology using IGBT, a method of using the IGBT together with a transformer has been known. However, this method entails serious disadvantages because of using the transformer that there are big limitations on the width of outputted pulse and the pulse rise/fall time. Further, it has other disadvantages that the entire apparatus is extremely large, has a low efficiency of operation, and has a very sophisticated circuit although it can protect the apparatus from arc strike. Both a method of using a Marx pulse generator and a method of using IGBT along with a transformer require the use of a high voltage charger, and in the case of using SCR there is a disadvantage that the entire apparatus becomes too big.
Accordingly, in order to overcome the above limitations, the present applicant and inventor filed a patent application entitled “pulse power generator using semiconductor switch” (Korean Patent No. 0820171 and U.S. Pat. No. 7,843,087). This patent (hereinafter, referred to as prior patent) discloses a pulse power generator, in which the lifespan is significantly improved, and the miniaturization and diverse control of a high voltage pulse finally outputted are enabled.
The pulse power generator of the prior patent includes a plurality of power stages connected in series with one another, each power stage including a plurality of power cells connected in series with one another, wherein each power cell has a semiconductor switch and an energy storage capacitor; a power inverter for supplying power so as to charge the energy storage capacitor; a power loop for allowing a power to be supplied to the respective power cells from the power inverter, the power loop being formed of a high-voltage insulating cable; a control inverter for supplying a control signal so as to generate a gate signal and a gate power of the semiconductor switch; and a control loop for allowing the control signal to be supplied to the respective power cells from the control inverter, the control loop being formed of a high-voltage insulating cable.
Here, the plurality of power stages are all connected in series with one another. Since the power cells inside the respective power stages are all connected in series with one another, all power cells inside the pulse power generator are connected in series with one another.
In this case, the respective power cells constituting the power stages include a semiconductor switch, for example, IGBT and an energy storage capacitor connected in series therewith. Also, the semiconductor switches and the energy storage capacitors of the whole power cell in the respective power stage are connected in series with one another. Thus, the semiconductor switches and the energy storage capacitors of the whole power stage constituting the pulse power generator are all connected in series with one another.
Also, each power cell is configured to include a bypass diode connected to both ends of the semiconductor switch, a rectifying diode connected to both ends of the energy storage capacitor, and a power switch driver (gate driving circuit) receiving a gate power insulated from the control loop of a single turn to apply the gate signal and the driving power for driving the semiconductor switch.
These power cells receive power for charging of the capacitor through the power loop connected from the power inverter, and receive the control signal through the control loop connected from the control inverter.
That is, each power stage includes a transformer constituted by the power loop and the control loop. When the inverter supplies a high voltage AC power through the power loop, the voltage adjusted by the power transformer is provided to each power cell to be charged into the capacitor. The control signal applied from the control inverter through the control loop is applied to the power switch driver to output the gate signal and the driving power.
In addition, in order to compensate for a difference of the charge voltage between the energy storage capacitors, the prior patent allows compensation windings to be insertedly connected between the power transformers of the upper and lower power stages so as to have a subtractive polarity. Thus, unbalance of charge voltage between the energy storage capacitors due to a difference of the leakage inductance between the transformers can be overcome.
On the other hand, in the pulse power generator configured as above, all the energy storage capacitors are charged in parallel, and then are connected in series through the switches. Thus, a high voltage pulse is generated by simultaneously discharging the energy storage capacitors in series.
In this case, while the generated high voltage pulse is being applied to the load, the high voltage pulse outputted to the load, as shown in FIG. 1, shows a voltage droop in which the voltage drops by a certain level every pulse.
The voltage droop occurs because the voltage of the energy storage capacitor gradually drops by the discharging to the load while the pulse is being applied with energy charged in the energy storage capacitor. The voltage droop may not matter according to the application field of the pulse power generator.
However, when the pulse power generator is used as a power generator for an electron accelerator or a pulse power generator for a radar, or is applied to fields of klystron, strict conditions are required.
FIG. 1 is a diagram illustrating a limitation according to a prior art. Here, t denotes time, and VMarx denotes a voltage of a high voltage pulse that is generated in a pulse power generator with a Marx structure and is finally applied to a load.
In this case, the amount of voltage droop (voltage drop) ΔVdroop is related to the capacitance CMarx, the pulse current Ipulse, and the pulse width tpw of the whole energy storage capacitor in the pulse power generator.
As shown in FIG. 1, a voltage droop occurs in every pulse generated while a high voltage pulse is being applied in the pulse power generator. In this case, the application fields of the pulse generator for an electron accelerator or a radar require a low degree of the voltage drop, i.e., the percentage voltage drop within a standard value.
Accordingly, there is a need for a method that can minimize the percentage voltage drop while showing a wide pulse width in a pulse power generator like the prior patent.
Although a method of significantly increasing the capacity of the energy storage capacitor can be considered as a method for reduce the voltage droop, a substantially impossible level of capacitor capacity is required to minimize the voltage droop. Particularly, when the capacity of the capacitor in the high voltage power supply increases, not only cost increases but also volume significantly increases. When the pulse width is lengthened, there is a limitation.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.